1. Field of the Invention
The present invention relates to a wireless selective calling receiver and, more specifically, to a wireless selective calling receiver which performs an intermittent receiving operation by selecting one of a plurality of receiving channels.
2. Description of the Related Art
A conventional wireless selective calling receiver performs intermittent receiving as shown in a process flow of FIG. 1. Specifically, after the power-on, a sync state flag SYNC and a channel control flag LOCK are initialized (step F1). Until a frame sync signal and a frame information signal (which can be subjected to error detection and correction) are detected normally, an intermittent receiving operation is performed while the receiving frequency is sequentially switched at given intervals (hereinafter referred to as "channel scanning") for pulling into synchronization (step F2). once a frame sync signal and a frame information signal are detected normally (steps F3, F5, and F6), an intermittent receiving operation is performed (step F102), together with channel scanning, in which reception is effected only at a predetermined frame position on each channel until network information (only the area information in FIG. 1) of the self receiver is detected (steps F12 and F13). If the network information of the self receiver is detected (Yes at step F13), the receiving frequency is fixed at the frequency of a network-information-detected channel (hereinafter referred to as "channel lock"; step F103). Subsequently, an intermittent receiving operation in which reception is effected at a predetermined frame position only on the fixed channel is performed (step F104).
However, in the above conventional technique, as shown in the process flow of FIG. 1, if the network information (only the area information in FIG. 1) of the self receiver is not detected (No at step F13) even once during the intermittent receiving operation in the channel lock state (step F104), the channel lock is canceled (step F101) and an intermittent receiving operation also comes to involve the other channels (step F102).
Therefore, there is a possibility that when the sensitivity of the receiver falls due to sudden occurrence of noise or temporary entrance into an area of poor reception of radio waves, a network information signal in which errors have occurred at more than an error detectable or correctable number of bits to become another correct code is received as if it were a correct signal. Even in this case, if the value of the received network information is found different even only once from the network information of the self receiver as a result of their comparison, the channel lock is canceled to start scanning the other channels. This is a factor of shortening the battery life. In addition, this may cause a problem that should locking be made to another channel erroneously (for instance, due to accidental conversion into incorrect reception data) while the other channels are scanned, it is no longer possible to receive data on the network channel of the self receiver.
In recent years, as one measure to accommodate the increasing transmission rate in the wireless selective calling signal scheme, a transmission technique of modulating and transmitting a 4-value signal comes to be employed instead of the previous signal scheme in which a 2-value signal is modulated and transmitted. Even in this scheme of transmitting a 4-value signal, each of a preamble signal that is used to detect an intermediate level of a signal and to take bit synchronization and a frame sync signal that is used to take frame synchronization is constituted of the lowest and highest levels of the four values, and hence is substantially equivalent to a 2-value signal. Therefore, in a situation that the sensitivity of the receiver is not high, during the channel lock, it is likely that although a frame sync signal is detected, ensuing network information and frame information, which are in four values, are detected erroneously (or as incorrect information caused by accidental conversion).